This invention pertains to the radiographic diagnostic arts and more particularly to a radiographic apparatus and method for dynamic enhancement of video image signals. The invention is particularly applicable to radiographic apparatus which produce a two dimensional shadowgraph of a three dimensional area of interest through the patient and which display the shadowgraph as video images.
Radiographic apparatus are known in which a patient is subjected to radiation from an x-ray source. The radiation which has traversed the patient is received on a fluoroscopic screen or the like for converting the radiation into an optical image whose intensity corresponds to the intensity of the received x-radiation. By placing photographic film adjacent the screen, a photograph of the image is produced.
Others have disposed a television camera to convert the optical image into a video signal, note for example U.S. Pat. Nos. 3,573,461 which issued Apr. 6, 1971 to S. A. Ohlsson, 3,582,651 which issued June 1, 1971 to M. P. Siedband, 3,784,816 which issued Jan. 8, 1974 to A. S. Abrahamsson and 3,848,130 which issued Nov. 12, 1974 to A. Macovski. The video signals from the camera are operated on by various signal and image processing means. The processing may include various additive and subtractive operations on the video signal.
The intensity of radiation transmitted through an object is an exponential function of the thickness and density of the object. That is, the intensity of radiation is attenuated as a function of the thickness and density of each subregion of the object traversed by the radiation. With respect to density of elements or subregions of the object, the subtractive and additive operations performed on the gray scale portion of the signal are nonlinear operations. This nonlinearity causes distortions in the processed image.
For medical diagnostic purposes, small changes in the level of the gray scale may be very significant. Television cameras, however, tend to lack stability in their gray scale amplitudes. That is, the reference (black or zero) intensity level of the gray scale tends to fluctuate while the camera is being operated. This causes errors and fluctuations in the gray scale of various parts of the video signal.